selftests: mptcp: userspace pm: skip if 'ip' tool is unavailable

selftests: mptcp: userspace pm: skip if 'ip' tool is unavailable

commit 723d6b9b12338c1caf06bf6fe269962ef04e2c71 upstream.

When a required tool is missing, the return code 4 (SKIP) should be
returned instead of 1 (FAIL).

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 259a834fadda ("selftests: mptcp: functional tests for the userspace PM type")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>